Vapor generator with integral economizer



. 16, 1969 v R. A. GREEN 3,483,848

VAPOR GENERATOR WITH INTEGRAL ECONOMIZER Filed Dec. 1, 1967 RICHARDALLEN GREEN, DECEASED BAMONA BERNICE GREEN, EXEcUTRix.

AT T RNEY United States Patent 0 3,483,848 VAPOR GENERATOR WITH INTEGRALECQNOMIZER Richard Allen Green, deceased, late of Gait, Ontario, Canada,by Ramona Bernice Green, executrix, Galt, Untario, Canada, assignor, bymesne assignments, to Babcocir & Wilcox Canada Ltd, Gait, Ontario,Canada, a corporation of Canada Filed Dec. 1, 1967, Ser. No. 689,747lint. Cl. F221) 1/02 1L8. Cl. 12232 13 Claims ABSTRACT OF THE DISCLOSUREA vapor generator wherein heat is extracted from a primary fluid flowingin tubes extending within a hollow vessel to vaporize a secondary liquidintroduced into the vessel, the inflowing secondary liquid being firstpreheated to a given temperature in an economizer chamber containedwithin the vessel by heat transferred from a portion of the overall tubesurface area, and the preheated liquid is passed into the volume of thevessel outside the economizer and comingled with recirculated secondaryliquid for vaporization by heat transferred from the remaining portionof the overall tube surface area. The economizer chamber is arrangedwithin the vessel in separated, spaced-apart relation to the internalboundaries thereof so that substantially no secondary liquid below thegiven preheat temperature contacts the internal boundaries of thevessel, thereby minimizing thermal shock to the vessel and its tubesheet, and promoting a more effective vaporization of substantiallyisothermal preheated liquid.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates ingeneral to vapor generators of the tube and shell type wherein heatextracted from a hot primary fluid flowing in tubes within the shellvessel is used to heat and vaporize a secondary liquid introduced intothe shell vessel. More particularly, the invention is directed to asteam generator wherein the inflowing feedwater, which is the secondaryliquid makeup, is first passed through an economizer chamber integrallycontained within an enclosed hollow vessel for preheating beforedelivery into the portion of the vessel outside the economizer, wheresuch preheated liquid together with recirculated secondary liquid isfurther heated and vaporized.

In accordance with the invention, preheating of the feedwater in theeconomizer is accomplished by heat transferred from a portion of thetotal effective surface area of the primary fluid carrying tubes, andvaporization of the preheated feedwater outside the economizer, butwithin the vessel, is accomplished by heat transferred from theremaining portion of the total effective tube surface area. Preferably,the economizer is so constructed and arranged with respect to the tubesurface area used for preheating, that the feedwater leaving theeconomizer is heated to saturation temperature for the pressure existingwithin the vaporization zone of the vessel.

The economizer chamber is arranged within the vessel in separated,spaced-apart relation to the internal boundaries thereof, and theinflowing feedwater enters the economizer through a thermally shieldedconduit that penetrates the vessel for communication with an externalfeedwater source. This serves to protect the vessel and tube sheet fromthermal shock, since all feedwater below the preheat temperature is heldwithin the economizer chamber, and substantially no liquid below thepreheat temperature contacts any portion of the internal boundaries ofthe vessel.

As to the heating of the feedwater in the economizer, it should be notedthat the general concept of the invention requires only that suchfeedwater heating be accomplished by heat transfer from a portion of theavailable tube surface area. It has been found that by arranging one ormore, or even all of the tubes to have length portions extending throughthe economizer, a relatively economical construction results. However,it should be noted that other tube arrangements can be used to providethe same intended feedwater heating.

For example, a group of tubes can be weld united along similar lengthportions to define the lateral enclosure wall or walls of theeconomizer, in which case the inwardly facing semicylindrical portionsof such tube lengths will be the feedwater preheating heat transfersurface area, and the outwardly facing semicylindrical portions of thesame tube lengths, together with other tube length portions notassociated with the economizer will be the vaporization heat transferarea.

The invention is readily adaptable for incorporation into preexistingvapor generator designs, as for example, that represented by US. PatentNo. 2,862,479 to Blaser et al.

According to a preferred embodiment of the invention, wherein U-shapedtubes are utilized to convey the primary fluid through the interior ofthe vessel, and length portions of such tubes extend through theeconomizer, the inlet and outlet of the economizer are arranged toprovide a feedwater flow path therethrough which is countercurrent tothe primary fluid flow in the tube length portions within theeconomizer. The economizer preferably provides a fecdwater flow pathwhich is dimensioned in relation to the effective surface area of thetube length portions inside the economizer to establish a net heattransfer rate between the primary fluid flowing through such tube lengthportions and the feedwater flowing along its preheating path, to raisethe temperature of the preheated feedwater delivered at the economizeroutlet to a value which exceeds the temperature of the primary fluidexiting the economizer in the tubes.

With the invention, it is thus possible to preheat the feedwater to asaturation temperature that is higher than the exit temperature of theprimary fluid, for a given combination of primary fluid and feedwaterflow rates, feedwater inlet temperature and primary fluid economizerentrance and exit temperatures.

Another important advantage afforded by the invention is that by reasonof the separation of the economizer chamber from the internal boundariesof the vessel, it becomes possible to circulate freely the saturatedliquid contained within the vessel along the inside wall surfacesthereof, and along the inside face of such tube sheets as are used inconjunction with the primary fluid tubes.

For example, with a vessel, U-tube bundle, tube sheet and downcomershroud arrangement similar to that as disclosed by the aforesaid Blaseret a1. patent, saturated liquid recirculated or returned through thedowncomer passage is directed to sweep across the tube sheet, flowing inthe passage defined by the clearance between such tube sheet and thebottom of the economizer.

By providing a vapor generator with an economizer design which allowssuch saturated liquid circulation, the thermal stresses acting upon thevessel and the tube sheet are reduced due to the more uniform boundarytemperature distribution effected by the circulation, the build-up ofdeposits on the vessel walls and the tube sheet is greatly reduced, andsuch circulation tends to promote a more effective vaporization of thesaturated liquid in contact with the vaporizing surface portion of thetubes.

It therefore is an object of the invention to provide a vapor generatorof the tube and shell type having an economizer integrally containedwithin the shell vessel for preheating the feed liquid prior tovaporization.

Another object of the invention is to provide a vapor generator asaforesaid wherein the feed liquid does not come in contact with eitherthe vessel wall surfaces or any tube sheet exposed to the hot primaryheating fluid until after such feed liquid has been preheated within theeconomizer.

A further object of the invention is to provide a vapor generator asaforesaid which allows free circulation of saturated liquid along thevessel Wall surfaces and across the tube sheet.

Still another and further object of the invention is to provide a vaporgenerator as aforesaid whereby the feed liquid can be preheated withinthe economizer to a saturation temperature higher than the exittemperature of the primary fluid from which has been extracted the heatrequired to eflect both preheating and vaporization of the feed liquid,as well as vaporization of a portion of the recirculated secondaryliquid.

Other and further objects and advantages of the invention will becomeapparent from the following detailed description and accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevation view, partly insection, of a vapor generator according to a preferred embodiment of theinvention.

FIG. 2 is a plan view of the vapor generator of FIG. 1 as taken lookingdownward along section line 2-2 therein.

FIG. 3 is an elevation view, partly in section, of an economizer chamberof different construction from that shown in FIGS. 1 and 2, and whichcan be substituted therefor.

FIG. 4 is a plan view, partly in section, of the economizer chambershown in FIG. 3, as taken along section line 4-4 therein.

DESCRIPTION OF THE PREFERRED EMBODI- MENTS OF THE INVENTION In FIGS. 1and 2, the vapor generator includes a vertically elongated pressurevessel 12 having a lower hemispherical head 16 closing the lower endthereof and a dished upper head 14. During normal operation of the vaporgenerator 10, there is a normal liquid level 22 which forms an uppervapor space 24 and a liquid space 26. Plate means including plates 28,30, 32 and 34 in conjunction with the walls of pressure vessel 12 coactto form a vaporization zone 36 and an annular downcomer passage 38, theplates 32 and 34 functioning as a shroud laterally surroundingvaporization zone 36 externally and being itself surrounded externallyby the wall of pressure vessel 12. Extending above the zone 36, capplate 28 and wall plate form a prismatic extension chamber 40 for thecollection of a vapor-liquid mixture.

Within vaporization zone 36 are disposed a multiplicity of U-shapedtubes 42 constituting a vertically elongated tube bundle. A tube sheet44 located transversely of the longitudinal centerline of pressureVessel 12 and secured thereto has a multiplicity of tube seats 45 formedtherein.

The ends of the U-shaped tubes 42 pass through the tube sheet 44 and aresecured in respective tube seats 45 to accommodate the flow of a hotprimary fluid through each tube 42 from the inlet end thereof to theoutlet end thereof. To aid and simplify the supply and return of theprimary fluid, the inlet ends of tubes 42 are inserted into the tubeseats 45 grouped together, as are the outlet ends of tubes 42 into tubeseats 45a. The hemispherical head 16 is fastened to the outer edge oftube sheet 44 to enclose the outer face thereof and form in conjunctionwith a dividing baflie 48 a primary fluid inlet chamber 50, and aprimary fluid outlet chamber 52, which respectively communicate with theinlet and outlet ends of tubes 42. Inlet and outlet nozzles 54 and 56respectively are disposed for connection to external primary fluidsupply and return means (not shown) to supply hot primary fluid to inletchamber and to withdraw spent primary fluid from outlet chamber 52.

Upright whirl chamber type vapor-liquid separators 58 are arranged in aring in the upper portion of pressure vessel 12 to receive avapor-liquid mixture which is emitted radially outward from the outlets60 of the vaporliquid collection chamber 40.

Each of the separators 58 is similar to those described in US. Patent2,862,479 to Blaser et al., these whirl chambers 58 being arranged sothat the normal liquid level 22 is positioned at the upper one-third oftheir respective heads. Each separator 58 has an outwardly directedextension piece 62 at its liquid discharge end arranged to direct theseparated liquid, which ordinarily is at saturation temperaturecorresponding to the pressure within vessel 12, into the downcomerpassage 38 adjacent the vessel 12 wall.

Separated vapor passes out the upper ends of the separators 58 intovapor space 24 and thence through the vapor outlet 68 of vessel 12 to apoint of use.

The makeup feed liquid enters the vessel 12 by a thermally shieldedinlet pipe 70, which communicates with the lower portion of aneconomizer chamber 72 disposed within the vessel 12 in separated,spaced-apart relation to the internal boundaries thereof to accommodatepreheating of the infiowing feed liquid to a predetermined temperature.T he economizer 72 has an outlet 74 communicating with the interior ofvessel 12, or more specifically, with the vapor generation chamber 36thereof. to deliver thereto preheated secondary liquid at theestablished preheat temperature, which temperature is preferably thesaturation temperature for the pressure prevailing within vessel 12.

The tubes 42 are in general arranged to transfer heat from a portion oftheir total effective surface area to the feed liquid in the economizerchamber to preheat such liquid to the intended economizer outlettemperature, and the tubes are further arranged so as to transfer heatfrom the remaining portion of their total effective surface area to thepreheated liquid within vaporization zone 36 to vaporize sarne.Consequently, respective portions of the total heat extracted from theprimary fluid which flows through tubes 42 are utilized to preheat thefeed liquid within the economizer chamber and to vaporize the preheatedliquid in vaporization zone 35, and therefore substantially no liquidbelow the preheat temperature contacts the internal boundaries of thevessel 12 or the zone- 36 side of tube sheet 44. For the embodimentspecifically shown by FIG. 1 the U-shaped tubes 42 are disposed ingenerally parallel relation to one another and convey the primary fluidin respectively similar directions from their individual inlet ends totheir outlet ends.

As can be noted from FIG. 1, the economizer chamber 72 has a pluralityof transversely extending baffles 76 which serve to define a sinuousfeed liquid flow path extending from the entrance of inlet pipe to theoutlet 74. The provision of such baffles 76 affords a more cificientutilization of the available tube heating surface allocated to feedliquid preheating.

By way of example, FIG. 1 shows the downcomer shroud plate 34 asdefining one of the lateral boundary walls of economizer chamber 72, avertical plate 78 defining another lateral boundary wall of saideconomizer chamber, and a plate 80 defining a bottom closure foreconomizer chamber 72, the downcomer shroud 34, vertical plate 78 andbottom plate 80 being joined together along their respectivelyintersecting edges to define a generally enclosed economizer chamber 72,the outlet 74 thereof being defined by the uppermost bafiie plates 76.The tubes 42 having length portions 42a within the economizer chamber 72extend through the bottom plate 30 and their intersections with saidplate 80 are sealed to retain the feed liquid within the economizerchamber 72. Bottom plate 80 is disposed in spaced-apart relation to theinside face of tube sheet 44 to define a clearance passage therebetweenwhich allows free circulation across the tube sheet 44 of preheatedliquid contained in zone 36 and the saturated liquid as is returnedthrough downcomer passage 38. In this way, the circulation of liquid atsaturation temperature along the inside wall surface of vessel 12, andalso across tube sheet 44 tends to prevent the accumulation of depositson such surfaces.

In accordance with the general concept of the invention, it is notabsolutely necessary that the tube length portions allocated forpreheating the feed liquid extend through economizer chamber 72, as isshown by FIG. 1, but it has been found convenient and relativelyeconomical to provide an arrangement of tubes 42 wherein at least oneand preferably a plurality of tubes 42 has a length portion 42aextending through the economizer chamber 72. If desired, one or more ofthe tubes 42 can extend through the interior of vessel 12 with allportions of its length being completely outside the economizer chamber72 as exemplified by tubes 42 shown in phantom, How ever, it isrecommended that all of the tubes 42 have similar length portions 42aextending through economizer chamber 72, so as to provide a reservepreheating surface capability in the event that during the service lifeof the vapor generator it should become necessary to plug one or more ofthe tubes 42. It should be noted that where all the tubes extend throughthe economizer chamber 72, the eflluent primary fluid from the severaltubes will be more uniform in temperature than in arrangements wheresome of the tubes are completely outside of the economizer chamber 72.

According to a preferred embodiment of the invention,

it is desired to achieve a liquid preheat temperature at the economizeroutlet 74 which is higher than the temperature of the primary fluid asmeasured at the tube 10- cations where the primary fluid flow exits theeconomizer chamber 72. This can be achieved by the economizer inlet 70and outlet 74 arrangement shown in FIG. 1 whereby the economizer chamberhas a feed liquid flow path which is countercurrent to the flow ofprimary fluid in the tube length portions extending through theeconomizer chamber 72.

The vapor generator 10 can be used to eflect vaporization of a widevariety of different feed or secondary liquids by means of heatextracted from any of a variety of hot primary fluids that flow throughtubes 42. Such primary fluid can be organic liquids, liquid metal, wateror gas, and can be derived from any suitable source having the heattransport characteristics and capability required to produce the desiredvapor conditions when using a feed liquid of given thermal conditions.In general, the vapor generator capabilities of vapor generator 10 will,for a fixed physical configuration, be dependent upon the flow and inlettemperatures of the primary and secondary fluids, and the internalpressure of vessel 12.

For a typical application of the invention as a steam generator, theprimary fluid can be heavy water with an inlet temperature of 560 F.pressurized to 1250 p.s.i.g., and the secondary liquid can be feedwaterat an inlet temperature 340 F, at 579 p.s.i.g., the internal pressure ofthe vessel 12. In view of the fact that the liquid in the vaporizationzone 36 will be in the equilibrium with the gen erated steam vapor, andhence is at the saturation temperature 484 F. for 579 p.s.i.g., and suchseparated liquid as is returned through downcomer passage 38 will be atsubstantially the same saturation temperature, it is advantageous toconstruct the economizer chamber 72 so that the feedwater delivered atits outlet 74 and into zone 36 is also at substantially saturationtemperature. In this way, all of the liquid contacting the wall ofvessel 12 and the inside face of tube sheet 44 will be at a uniformtemperature, namely the saturation temperature of 484 F. As can beappreciated by the artisan, maintaining the inside of vessel 12 and tubesheet 44 at a uniform temperature is advantageous in reducing thermalstresses that would otherwise exist if the inside boundary surfacetemperature were allowed to vary appreciably from pointto-point.

The economizer chamber 72 and its baflles 76 are preferably sodimensioned and arranged to provide a feedwater flow path therein ofcross-sectional and length dimensions selected in relation to theeffective surface area of the tube length portions 42a inside economizerchamber 72 to establish a net heat transfer rate between the primaryfluid flowing through such tube length portions 42a and the feedwatertraveling along its flow path as to raise the temperature of thepreheated feedwater delivered at outlet 74 to the desired temperature,which can be any temperature above the feedwater inlet temperature up tosaturation temperature, and which is preferably the saturationtemperature. Such designing of the economizer chamber 72 incorporatingtube length portions 42a and the baflles 76 can be accomplished by thestraightforward application of routine engineering calculations andformulas, all of which are presumed to be well known to the artisan. Theinlet pipe and economizer outlet are so arranged that the feedwater flowduring preheating is countercurrent with respect to the primary fluidflow in the tube length portions 42a extending through economizerchamber 72, or whatever tube length portions are utilized for preheatingpurposes. With countercurrent flow, the feedwater can be readilypreheated to an outlet temperature above the temperature at which theprimary fluid exits the economizer 72, as measured at the tube locationsintersecting with bottom plate 80.

While with a fixed vapor generator 10 configuration, the feedwateroutlet temperature, and primary fluid exit temperature will be dependentupon the flow rates and inlet temperatures of such two fluids, it alsofollows conversely that the economizer chamber 72 can be sized togetherwith the tubes 42 so as to produce steam vapor having saturationtemperature and pressure properties within an acceptable range for caseswherein the steam demand varies, and/or other operating parameters vary,assuming that suitable external control means (not shown) are providedto regulate the associated pressure as well as the feedwater flow rate,primary fluid flow rate, feedwater inlet temperature, and primary fluidinlet temperature to the extent necessary to compensate for suchvariations.

As a guide to the artisan in understanding and practicing the invention,the typical design parameters of a steam generator constructed inaccordance with the FIGS. 1 and 2 embodiment of the invention are setforth in the following Table I.

TABLE I Steam generation flow rate lb./hr 540,000 Steam conditions(saturated):

Pressure p.s.i.g 579 Temperature F 484 Enthalpy B.t.u./lb 1203 Feedwaterconditions:

Inlet temperature F 340 Pressure (same as steam) p.s.i.g 579 Primaryheating fluid operating conditions:

Primary fluid Heavy water Pressure p.s.ig 1250 Temperature Inlet F 560Outlet F 480 Flow rate lb./hr 5.111 10 Primary fluid tubes (0.496 OD0.049 wall):

Number of tubes 2600 Total heating surface sq. ft 20,000 Economizerheating surface sq.ft 2650 vaporization zone heating surface sq. ft17,350

In connection with the foregoing example, it should be realized that inthe event that it should be desired to construct a steam vapor generatorin accordance with the invention to produce steam at a different flowrate, or outlet thermal conditions, or to utilize different primaryfluid operating conditions, such can be done by modifying the dimensionsof the basic vapor generator 10 as dictated by conventional engineeringpractice.

It has been found that one of the advantages afforded by the integraleconomizer 72 provided by the invention, as compared with direct contactfeedwater heating, lies in the fact that with the economizer 72, about20 percent less overall heating surface is required than for directcontact feedwater heating, i.e. without an economizer 72. Furthermore,with direct contact feedwater heating, the primary fluid outlettemperature establishes the maximum saturation temperature and hence themaximum steam operating pressure, whereas with the invention, thecountercurrent feedwater flow through the economizer allows a feedwaterpreheat temperature higher than the primary fluid outlet temperature,and therefore simply by sizing the economizer tube heating surface areato preheat the feedwater to saturation, a sizeable boost in attainablesteam pressure can be realized.

FIGS. 3 and 4 serve to illustrate that the concept of the invention isnot necessarily limited to the economizer 72 construction specificallyshown in FIGS. 1 and 2 but that other economizer configurations can besubstituted. From FIGS. 1 and 2, it will be recalled that the lateralboundaries of the economizer 72 are defined by a portion of thedowncomer shroud 34 and a vertical plate, since such members areconveniently available. However, as shown by FIGS. 3 and 4, the lateralboundaries of the economizer chamber 172 can be formed by a plate 178and an array of circumferentially spaced primary fluid carrying tubes150 which are weld-united along portions 150a of their lengths by fillerbars 143, the inward semicylindrical surfaces of the tube portions 150a,together with similar length portions 142a of the tubes 142 passingthrough the interior of economizer 172, constituting the surface areaallocated to feedwater preheating. The outward semicylindrical surfacesof tube portions 150a, together with such other tubes 142 and lengthportions thereof as are outside of economizer chamber 172 serve as thevaporization or boiling heating surface area.

The bottom of economizer 172 is closed by a plate 180 which is similarto the plate 80 shown in FIGS. 1 and 2, and a plurality of bafiies 176are supported by the tubulous wall defined by tube length portions 150aand filler bars 143, and/or by plate 178.

As can be appreciated by the artisan from the foregoing, the inventionis susceptible of numerous modifications and variations as will becomeobvious therefrom. However, the invention is intended to be limited onlyby the following claims in which I have endeavored to claim all inherentnovelty.

What is claimed is:

1. A vapor generator which comprises a substantially enclosed hollowvessel having an outlet for the discharge of vapors, a plurality oftubes extending through the interior of said vessel to conveytherethrough a hot primary fluid from which heat is extracted to heatand vaporize a secondary liquid, and means defining a generally enclosedeconomizer chamber disposed within said vessel to accommodate preheatingof an inflowing secondary liquid to a predetermined temperature, saideconomizer chamber having an inlet disposed to receive secondary liquidfrom an external source thereof, and an outlet communicating with theinterior of said vessel to deliver thereto preheated secondary liquid atsaid temperature, said economizer chamber being spaced-apart from theinternal boundaries of said vessel to define therewith flow spaces forthe free circulation of preheated secondary liquid in contact with theentire exterior surface of the economizer chamber, said tubes beingdisposed to transfer heat from a portion of their total effectivesurface area to the secondary liquid in said economizer chamber topreheat said liquid to said temperature, and to transfer heat from theremaining portion of their total effective surface area to the preheatedsecondary liquid in said vessel to vaporize same, whereby respectivelycorresponding portions of the total heat extracted from the primaryfluid are utilized to preheat the secondary liquid within the economizerchamber, and to vaporize the preheated secondary liquid within thatportion of the vessel outside the economizer chamber, and substantiallyno secondary liquid below said reheat temperature contacts the internalboundaries of said vessel.

2. The vapor generator according to claim 1 wherein said tubes arearranged to provide a surface area portion disposed in heat transferringrelation to said economizer chamber to preheat said inflowing secondaryliquid to a temperature corresponding to the saturation temperature forthe internal pressure of the vessel, for a given combination ofsecondary liquid flow rate, secondary liquid inlet temperature, primaryfluid flow rate, and primary fluid tube inlet temperature conditions.

3. The vapor generator according to claim 1 wherein at least one of saidtubes has a length portion extending through said economizer chamber.

4. The vapor generator according to claim 1 wherein at least one of saidtubes extends through the interior of said vessel with all portions ofits length being completely outside said economizer chamber.

5. The vapor generator according to claim 1 wherein said tubes aredisposed in generally parallel relation to one another and convey saidprimary fluid in respectively similar directions from their individualprimary fluid inlet ends to their respective primary fluid outlet ends,and wherein a plurality of said tubes having correspondingly similarlength portions extending through said economizer chamber.

6. The vapor generator according to claim 5 wherein the inlet and outletof said economizer chamber are arranged, in relation to the primaryfluid inlet and outlet ends of the tubes having length portionsextending through the economizer chamber, to define a secondary liquidflow path through the economizer chamber which is countercurrent to theflow of primary fluid in said tubes.

7. The vapor generator according to claim 6 wherein said economizerchamber has a secondary liquid flow path dimensioned in relation to theeffective surface area of the tube length portions inside the economizerchamber to establish a net heat transfer rate between the primary fluidflowing through said tube length portions and the secondary liquid alongsaid flow path to raise the temperature of the preheated secondaryliquid delivered at the outlet of said economizer chamber to a valuewhich exceeds the temperature of the primary fluid as measured at thetube locations where the primary fluid flow exits the economizerchamber, for a given combination of primary fluid flow rate, secondaryliquid flow rate, primary fluid economizer chamber entrance and exittemperature, and secondary liquid inlet temperature conditions.

8. The vapor generator according to claim 6 wherein said tubes areU-shaped and extend in generally vertical planes, and including a shrouddisposed within said vessel in external laterally surrounding relationto said tubes and in spaced-apart internally surrounding relation to theinternal boundary wall surface of said vessel to define therewith anannular downcomer passage extending along said wall surface a portion ofsaid downcomer shroud defining a boundary portion of said economizerchamber.

9. The vapor generator according to claim 8 including a tube sheetdisposed within said vessel to receive and support said tubes at theirprimary fluid inlet and outlet extremities, said tube sheet beingdisposed in spacedapart relation to said economizer chamber and inspacedapart relation to the lower end of Said downcomer shroud J toallow free circulation over said tube sheet of preheated secondaryliquid contained in said vessel and saturated secondary liquid returnedthrough said downcomer passage.

Til. The vapor generator according to claim 9 wherein said economizerchamber has a bottom closure plate disposed in adjacent spaced-apartrelation to said tube sheet, and a vertically extending lateral closureplate disposed within said downcorner shroud and connected edgewisethereto, said bottom closure plate being connected along its edges tosaid downcomer shroud and lateral closure plate to define an economizerchamber enclosed at its bottom portion and laterally and having anoutlet at its upper portion.

11. The vapor generator according to claim 10 including a thermallyshielded secondary liquid inlet conduit extending through the wall ofsaid vessel and said downcorner shroud for communication with saideconomizer chamber at the lower portion thereof.

12. The vapor generator according to claim 11 wherein it? the tubeshaving length portions extending within said economizer chamber passthrough the bottom closure plate thereof in sealing engagementtherewith.

13. The vapor generator according to claim 12 including a plurality ofbafile plate members disposed within said economizer chamber to increasethe eifective secondary liquid preheating flow path therein.

References Cited UNITED STATES PATENTS 3,141,445 7/1964 Bell 122-343,356,135 12/1967 Sayre 12232 XR 2,220,045 10/1940 Kraft et al. 122-32FOREIGN PATENTS 84l,800 1/1960 Great Britain.

KENNETH W. SPRAGUE, Primary Examiner

